Piston rings normally are made to fit in an annular groove in pistons and to bear against the walls of the cylinders in which the pistons run to make a sliding seal. Piston rings characteristically are made of springy metal. However, metal piston rings require lubrication and in uses where oil cannot be tolerated metal rings cannot be used. Oil cannot be used for compressing medical or chemically pure gases or in compressing oxygen, for example. In such uses self-lubricating rings must be used.
One excellent self-lubricating material is polytetrafluoroethylene which is also known as Teflon. Teflon not only is self-lubricating but it also withstands the heat that is generated by friction. Lubricated metal piston rings experience practically no wear. However, the self-lubricating quality of Teflon at least in part is provided by wearing away of the surface of the Teflon. In use, the high pressure side of the cylinder will tend to push the piston ring toward the low pressure side of the annular piston ring groove. This action permits high pressure to get behind the piston ring. The slight leakage between the piston ring and the cylinder wall creates a relatively low pressure on the exterior of the piston ring and the result of the pressure difference thus created is that the piston ring will be pushed tightly into contact with the cylinder wall. This action promotes good sealing but it also promotes piston ring wear.
One form of piston ring is a spiral piston ring. A piston ring made in the form of a spiral with one or more convolutions having abutting adjacent surfaces can be placed in the annular groove of a piston easily and it acts as a good seal against the cylinder wall. Spiral piston rings are disclosed in such early patents as U.S. Pat. No. 727,311 issued to Douglas et al., U.S. Pat. No. 1,165,084 issued to Flammang, U.S. Pat. No. 1,711,244 issued to Newton, and U.S. Pat. No. 1,819,890 issued to Gleason.
Since piston rings must bear against the walls of the piston ring groove in which they ride they must have square ends. With spiral piston rings this is accomplished in different ways. In the Gleason patent mentioned above the center convolutions are made with abrupt, step-like changes in shape to accomodate the square ends of the top and bottom convolutions. These changes in shape are difficult and expensive to machine. In addition, although with metal piston rings there is no sacrafice in function or strength by having step-like configurations, Teflon made in that way frequently experiences failure at the steps.
Other spiral rings are made with long tapered end portions such as those disclosed in the Flammang patent. The tapered end portions disclosed in the Flammang patent taper around about 180.degree. of the angle of rotation of the cylinder and that gradual taper results in a sharp, almost knife-like end to the ring. To deal with these sharp ends the Flammang patent discloses an additional machining process to create a notch in the adjacent convolution of the helix to receive the end of the helix and a further step of cutting off the sharp end of the helix.
A sharp end at the end of a helix is dangerous when dealing with metal rings but not when dealing with rings made of Teflon. However, when the end of a Teflon ring is feathered too gradually a weak point is created that lends itself to being broken off in use.